1) Communication needs to get much better. Nobody cares about your "hip and funny" weekly updates which say nothing.
Nobody wants to be kept in the dark, concerning future developments.
2) Complete separation between PvE and PvP balancing.
3) No more gear resets.
4) No time gated quests.
5) No tripple layered RNG.
6) No cut content.
7) No 20€ expansions which aren't even worth half as much (this means that expansions need much more content in D2).
8) No more rotating PvP metagames.
9) No more ridiculously bad endgame loot/rewards (Nightfalls anyone?)
10) No non-existent story.
11) Reforge and etheric light (I don't care how they'll call it, it's about the principle) needs to come back.
12) Bring back elemental primaries/elemental infusion.
13) Optional matchmaking for endgame PvE and PvP activities.
14) In game LFG.
15) More social aspects (most obvious would be a vastly increased number of possible guardians within a Zone).
16) Dedicated servers and dedicated ranked playlists.
17) A mostly balanced PvP environment in which every weapon is viable.
18) Much more frequent balancing patches.
There needs to be at least one per month.
19) Dynamic weather and gravity conditions matching the environment (obv you can't go overboard with both of them).
20) Clan wars.
21) A huge dark zone á la the division.
22) 60 FPS at 1080p. Nothing else would be adequate.
I'm sure I forgot quite a few things though.
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That's a great starting list.